The continued development of metal-oxide-semiconductor field-effect transistors (MOSFET) has improved the speed, density, and cost per unit function of integrated circuits. One way to improve transistor performance is through selective application of stress to the transistor channel region. Stress distorts or strains the semiconductor crystal lattice, which affects the band alignment and charge transport properties of the semiconductor. By controlling the magnitude and distribution of stress in a finished device, manufacturers can increase carrier mobility and improve device performance. There are several existing approaches of introducing stress in the transistor channel region.
One conventional approach includes forming an epitaxial, strained silicon layer on a relaxed silicon germanium (SiGe) layer. Since the SiGe lattice is larger than Si, the SiGe layer stretches the epi-layer in the lateral direction, i.e., the silicon will be under a biaxial tensile stress. Such a stress applied to a MOS channel region is particularly effective in improving N-channel transistor performance.
With a PMOS device, a tensile stress improves performance when it is perpendicular to the channel, but it has nearly the opposite effect when it is parallel to the channel. Unlike an N-channel transistor, when a biaxial, tensile stress is applied to a PMOS channel, the two stress effects almost cancel each other out. Improved PMOS fabrication therefore includes using substrate structures that apply a compression stress to the channel. One PMOS method includes selective application of a SiGe layer within the source/drain regions.
A problem with the prior art is that the new materials and methods required of strain-engineered devices creates significant process integration issues. For example, a silicon nitride tensile film formed over a NMOS transistor may be used to improve carrier mobility. For PMOS devices, on the other hand, an embedded SiGe stressor may be formed in a silicon substrate. A problem with these approaches, however, is that using different materials at different stages of the fabrication process further complicates an already complex process. Therefore, there remains a need for using strain engineering to improve device performance without significantly adding to the cost or complexity of the manufacturing process.